Apparatus for extrusion of plastic net

ABSTRACT

An extrusion apparatus, having two die members which are moved relative to one another to extrude a plurality of spaced strands of plastic material in one direction and a plurality of spaced strands in a second direction to form an integral net or net-like structure is operated by generating a first electrical signal having a wave form correlated to a desired motion of the die members, monitoring the actual motion of the die members and generating a second electrical signal having a wave form corresponding thereto, comparing the first and second signals and generating a variable third signal adjusted to compensate for detected differences between the first and second signals, and transmitting the third signal to a torque motor-servo valve combination which controls flow of oil to a hydraulic cylinder piston assembly operatively connected to one or both of the die members for carrying out the desired motion thereof. The motion is carried out with a high order of precision and reproducibility enabling the extrusion of a wide variety of sizes and shapes of plastic net or similar structures.

United States Patent 11 1 Gaffney et al.

1451 July 31,1973

1 1 APPARATUS FOR EXTRUSION OF PLASTIC NET Inventors: Bernard J.Gaffney, Stillwater;

Ronald L. Larsen, Minneapolis, both of Minn.

Related US. Application Data Primary Examiner-Richard B. LazarusAtt0rneyEyre, Mann & Lucas [5 7 ABSTRACT An extrusion apparatus, havingtwo die members which are moved relative to one another to extrude aplurality of spaced strands of plastic material in one direction and aplurality of spaced strands in a second direction to form an integralnet or net-like structure is operated by generating a first electricalsignal having a wave form correlated to a desired motion of the die mem-Division 1969 bers, monitoring the actual motion of the die membersabandoned and generating a second electrical signal having a wave formcorresponding thereto, comparing the first and [52] Cl 425/150 425/162425/381 second signals and generating a variable third signal ad gfjusted to compensate for detected differences between 1e 0 care thefirst and second s1gnals, and transmitting the th1rd 425/167, 375, 381,155, 150; 264/40 signal to a torque motor-servo valve combination whichcontrols flow of oil to a hydraulic cylinder piston [56] References andassembly operatively connected to one or both of the UNITED S S PATENTSdie members for carrying out the desired motion 3,002,615 10/1961Lemelson 425/381 X thereof. The motion is carried out with a high orderof 3,252,181 5/1966 l-lureau 425/224 precision and reproducibilityenabling the extrusion of 3,422,648 1/1969 Lemelson-m a wide variety ofsizes and shapes of plastic net or simi- 3,480,999 12/1969 Carlo 425/150x structures 3,647,309 3/1972 Thompson 425/150 X 4 Claims, 14 DrawingFigures HYDRAULIC CYLINDER 64 TORQUE SERVO I F 68 uuenou j. GENERATORLIPLIFIER 89 01c. SIGNAL -'90 '42 FUNCTION PRESSURE GERggATOR Tulsataut-TOR PRER'URE 88 REGULATOR 0.0. SIGNAL it on. eoouue OIL UNITRESERVOIR COOLING WATER Patented July 31, 1973 4 Sheets-Shut 1 HYDRAULIC78 cvuuoca 64 10mm: ssnvo 50 1 l MOTOR\ VALVE A 74 28 FUNCTION A N -80GENERATOR 44 0.0.

m 70 SERVO 52A 4.. ouswucsusnr AMPLIFIER rnausoucan 1 o. c. SIGNAL -90 nEXTRUSION new FuNcnon TIMER PRESSURE 1 REGULATOR E'Z I PRESSURE aeeuuron88 34 67 U ,nmw

56 0.6. SIGNAL 1 5- 1 5 on. coouue OIL T I I I I I I I n um'r .4 32COOLING WATER FIG.

Patented July 31, 1973 4 Sheets-Sheet 2 SIGNAL A CIDUCJDEIUDCIDDCIUUUUUUUUUUUUUU DIRECTlON' 0000000000000 SIGNAL B SIGNAL A SIGNAL BPatenied July 31, 1973 4 Sheets-Sheet if -s|eNAl. A"- SIGNAL 8 FIG. 6

7 H A B A B L m N N A N N N N G m w mm 2 S S S S 2 UDnD H 0B UDDD U DOUUQU U 00 UUQUHU m .UUDD U B BUD U 00 BUD H 0B DUB U DD DUB NU 0 BUD H0B UmD H 0B USU an DUO US a a EXTRUSION DIRECTION SIGNAL B SIGNAL A D DD D D D U DDDDDCIUUEIDDUU DUOUDDUDDDCIUU DEIDDCIUCIDDCIDDC]UUDCJDUDUDDUDU DEJUUUDUUUUDUD UDUUUEJUDUDDDE] UEIUDUCIUGDUDUD EXTRUSIONDIRECTION Patented July 31, 1973 3,749,535

4 Shoots-Shoot 4.

EXTRUSION WRECTION 000000000000 FIG. I! 000000000000 000000000000 l000000000000 000000000000 DUUDUUUUUUUU 000000000000 000000000000000000000000 'G- I2 I 000000000000 :7

DEJDDDUEIUEJUUD EXTRUSION D'RECTON DUDDUEIDDDUDD EXTRUSION DIRECTIONEIUUUUDDUUUUCJ DDUUDUDUUUDD DUUDDDCIUDDUC] UUDEIEIDDDUUUU APPARATUS FOREXTRUSION F PLASTIC NET This is a division, of application Ser. No.877,686, filed Nov. 18, 1969 now abandoned.

In US. Pat. No. 3,252,181 and in US. Pat. No. 3,3 84,692, various formsof apparatus are described for extrusion of plastic tubes, plastic netand net-like structures. Some of these apparatuses include two coaxialdie members which may be reciprocated between two different positions.In the so-called closed position, coacting cooperative surfaces of thedie members are in contact with each other and in this position aplurality of spaced strands of plastic may be extruded through groovesin the face of one of the die members. In a second so-called openposition, the two die members are separated to extrude an annular strandof plastic that is integral with the spaced strands and arrangedtransverse thereto. By reciprocating the die members at speeds up to onehundred cycles per second a net-like structure is formed. The structuremay be in the form of an open network of strands or it may be a solidsheet reinforced with one or more of the strands. A wide variety ofstructures may be formed depending upon the construction of the die andcontrol thereof. In some cases, one or more of the die members may berotated or oscillated and simultaneously one or more of the die membersmay be reciprocated.

In operating such apparatus to produce net-like products havingspecified dimensions, it is important to have precise control of themotion of the die members. The length of the spaced parallel strands ofplastic between the transverse strands depends primarily on the timethat the die members are maintained in the closed position and there arealso secondary effects on the thickness or cross sectional area of thesestrands caused by the variable orifice conditions. The cross sectionaldimensions of the transverse strands depends on the displacementdistance between the two die members which controls the transversestrand radial thickness and the time the members are maintained in openposition, which controls the extrusion direction length of thetransverse strands. Therefore, properly dimensioned net products can bemade if the cyclical or reciprocal displacement motion is carried outwith a high order of precision and reproducibility. Moreover, in orderto manufacture uniform relatively fine mesh plastic net similar towindow screening, the reciprocal or cyclical motion may have to becarried out at a frequency of up to 40 cycles or more persecond. Withsuch high frequencies, the problems of maintaining a controlleddisplacement motion are compounded, since the margin for tolerabledeviations in the displacement motion and consequently in the product isextremely small, and the forces exerted and to be controlled are largeand have abrupt peaking values.

The words cycical and reciprocal are used interchangeably in thespecification and claims and either word refers to both uniform andnon-uniform displacement motion.

The present invention provides an apparatus and method for carrying outvery precise reciprocal or cyclical displacement motion in extrusionapparatus of the types described above. More particularly, the inventioninvolves use of a hydraulic cylinder and a double faced piston thereinfor directly controlling the displacement motion of one or two diemembers and means for controlling the flow of oil or otherpressuretransmitting fluid into and out of the hydraulic cylinder inresponse to electric signals having wave forms correlated to the desiredreciprocating motion. In a particularly desirable embodiment of theinvention, a first signal is generated having a wave form whichcorrelates to a desired reciprocating displacement motion by the diemembers, and a monitoring means is employed to track the actual motionof the members and generate a second signal having a wave form whichcorresponds to said actual motion. The first and second signals arecontinuously compared and a variable third signal is generated whichcompensates for any detected differences between the first and secondsignals, the compensation tending to eliminate such differences andconform the actual motion of the die members to that dictated by thefirst signal. This continuously adjusted third signal is transmitted toa control valve'which controls the flow of oil into and out of thehydraulic cylinder, which is in a hydraulic loop containing a reservoir,pump, regulators and conduits for providing the operative oil to thehydraulic cylinder.

If desired, the cyclical motion of the die members may be made such thatthe die members are always in open relationship to produce a tube orsheet of polymer having a varying cross sectional thickness dicatated bythe programmed movement of the die members.

Further details of the invention will be readily understood by referenceto the accompanying drawings which illustrate various embodimentsthereof and in which:

FIG. 1 is an overall view of the preferred system for extruding plasticnet-like structures in accordance with the invention;

FIG. 2 is a graph showing the wave form of a first signal which isgenerated and utilized in the system of FIG. 1;

FIG. 3 is another graph showing the wave form of a second signal whichis generated and utilized in the system of FIG. 1;

FIG. 4 is a graph showing the wave form of another signal which may begenerated and utilized in a particular mode of operation of the systemof FIG. 1;

FIG. 5 is a side view of a portion of plastic net-like product which maybe manufactured when the system i of FIG. 1 is operated with the waveform of FIG. 4;

FIG. 6 is a graph showing the wave form of still another signal whichmay be generated and utilized in a particular mode of operation of thesystem of FIG. 1;

FIG. 7 is a side view of a portion of plastic net-like product which maybe manufactured when the system of FIG. 1 is operated with the wave formof FIG. 6.

FIGS. 8-14 are side views of portions of plastic netlike products whichcan be manufactured when the system of FIG. 1 is operated in thealternate mode of operation referred to in connection with FIG. 8.

Referring now toFIG. 1, an extrusion apparatus of the type-disclosed inUS. Pat. No. 3,252,181 is there denoted by reference numeral 10. Thisapparatus in cludes a first stationary die member 12 in the from of aring having an operative surface 14 which slants inwardly from the topto bottom thereof. The member 12 is provided with a plurality of grooves15 spaced apart equal distances 360 around the entire die member. Thesegrooves provide channels for flow of spaced streams of molten plasticmaterial which exit from the lower ends of the grooves and form spacedvertical longitudinal strands 16. The apparatus further includes asecond die member 18 which is in the fonn of a solid circular platehaving an operative surface 20 which also slants inwardly from the topto the bottom thereof, matching the slant of operative surface 14 of thedie member 12. When member 18 is in a down or closed position, itsoperative surface 20 seats upon the operative surface 14 of member 12,whereby the spaced vertical longitudinal strands 16 exit from the die.When member 18 is elevated to a higher or open position, a continuousopen annulus is formed between the operative surfaces 20 and 14 ofmember 18 and 12, which annulus is in communication with all of thegrooves in the ring member 12. Therefore, in this position of themembers, an annular transverse strand of molten plastic exits from thecontinuous open annulus to form the single transverse strand 22 which isintegral with and, in effect, a transverse continuation of the spacedlongitudinal vertical strands 16. The apparatus also includes a polymerinlet 24 through which molten plastic material is pumped into theinterior of the apparatus, which is provided with suitable heating means(not shown) for maintaining the material in molten flowable condition.As member 18 is periodically lowered and raised into and out of contactwith the ring member 12, a plastic net or net like structure is formedby the resulting reciprocal displacement motion which alternately allowsthe single transverse strands 22 to exit from the die integral with thecontinuous longitudinal strands 16.

in accordance with the invention, member 18 is attached to a shaft 26which passes up out of extruder and into a hydraulic cylinder 28. Theshaft 26 is attached to a double-faced piston 30 disposed within thecylinder 28, with the periphery of the piston sealed in conventionalmanner in fluid tight relation with the inside wall of the cylinder.

A tank 32 holds a supply of oil or other pressure transmitting fluid. Anelectric motor 34 drives a hydraulic pump 36 which pumps oil out ofreservoir 32 and into conduit 38. The pump 36 is provided with ascrew-adjustable regulator 40 whereby the output pressure of pump 36 isset at a steady value of, for example, 2000 psi. A conduit 42 branchesfrom conduit 38 to deliver oil:under pressure of pump 36 into the servovalve 44 (Type SA 4 Series available from Vickers Incorporated). Conduit38 is provided with another pressure regulator 46 which may bescrew-adjustable to provide an output into conduit 48 of oil at a lowerpressure, for example 1,000 psi, as compared to the pressure in conduit42. Conduit 48 delivers the oil at lower pressure also into servo valve44.

Servo valve 44 is connected to two conduits 50A and 52A. Conduit 50Acommunicates with the space 503 above the top face of piston 30, andconduit 52A communicates with the space 528 below the piston 30. Conduit54 provides a return path of the oil pumped to the servo valve 44 viaconduit 42, this return path leading into a heat exchanger 56 by whichthe oil may be cooled before it is emptied into the reservoir 32 bymeans of a cooling medium circulated through the cooler 56 via the inputand output conduits 58 and 60 respectively. Conduit 62 provides a returnpath for the oil pumped at lower pressure into the servo valve viaconduit 48 and empties directly back into the reservoir 32.

The foregoing description relates to the hydraulic loop that connectsreservoir 32, servo valve 44 and the hydraulic cylinder 28 in an endlesspath.

Referring now to the system used to generate signals which control thehydraulic means for oscillating the die member 18, numeral 64 designatesa conventional function generator which is electrically powered by theoridinary 120 volt 60 cycle source to generate an adjustable signalhaving a periodic wave form such as illustrated in FIG. 2. In thisparticular figure, the wave form defined by voltage values (ordinate)versus time (abscissa) is a sine wave generated by the functiongenerator 64.

The sine wave shown in FIG. 2 constitutes one cycle of operation and inoperation the generator 64 is set at the desired wave form and cyclefrequency. Signal generator 64 is capable of generating signals up to1,000,000 cycles per second but the practical upper limit of operationof the extrusion apparatus is about cycles per second. The amplitude ofthe wave of function generator 64 can be varied by a DC potentiometer(not shown) or by any other conventional means.

In addition to the function generator 64, there is a D.C. generator 66by which voltage is added to or subtracted from the wave form signaldelivered by generator 64 and thereby the position of the wave can beshifted up or down without changing its shape or frequency as indicatedby the dotted line position of the abscissa in FIG. 2. The D.C.generator 66 merely changes the bias of the wave form to produce aposition change but not a shape or frequency change. In effect, theadded or subtracted DC. voltage shifts the wave and abscissa up and downto the dotted line position of the abscissa in relation to the ordinate.The position of the abscissa with respect to the closed position of thedie members 12 and 18 determines the relative amount of time the dieswill be open and closed. If the abscissa corresponds to the position inwhich the die members are just barely closed, the curve is shifted bymeans of D.C. generator 66 to bring the abscissa into the position wherethe die members are just barely closed and in such case operation of thedie members will follow the program of the wave form. The open time isrepresented by interval 660 on the sign curve of FIG. 2 and the closedtime is represented by interval 66b.

In certain applicatins, it may be desirable to depress the sign curve bymeans of D.C. generator 66 to bring the abscissa to a point below thedie closed position. This will result in decreasing the amplitude of thedie member in its open position and it will decrease the open time ofthe die member 18 relative to its closed time. The amplitude of diemember 18 is a function of the selected wave form and the amplitudedetermines the radial thickness of transverse strands 22.

In order to vary the open-closed time relationship of the die members bymeans of DC generator 66 without changing the radial thickness of thetransverse strands 22, the potentiometer (not shown) of functiongenerator 64 is adjusted to change the signal wave amplitude to achievethe desired strand radial thickness. Conversely, by raising the signcurve to bring the abscissa to a point above the closed position of thedie, the open time of the die members will be lengthened relative to theclosed time.

The function generator 64 may be set in known manner to generate anydesired wave form such as square waves, triangular waves, sawtoothwaves, and ramp or half waves may also be used. The ramp and sawtoothwaves are of advantage in that this form will give a slower openingmovement of die member 18 than that provided by the sine wave and theramp and sawtooth waves give a very quick closing movement. Sincepressure on the plastic material is relatively high when the die startsto open and relatively low during the closing movement, a slow openingand quick closing tends to balance the extrusion operation.

If desired, function generator 64 may be replaced by a function as aconventional curve following signal generator which will produce anelectric signal in accordance with the followed curve. In this manner,any type of curve may be electrically reproduced and therefore any typeof displacement motion of die 18 may be produced to achieve net productshaving heretofore unknown shapes.

Generator 64 may also be adapted to function as a variable periodgenerator which in known manner may separately vary the period andamplitude of the portion of the wave form that controls the open time ofthe die members and separately the portion that controls the closed timeof the die members.

A second function generator which produces a trigger signal may be usedin known manner in conjunction with function generator 64. If thetrigger signal is a square wave form of 20 cps. and the functiongenerator 64 is set to produce a single wave comprised of half sine waveplus an equal period zero voltage with a predetermined amplitude at apredetermined frequency, say 40 cps. then 20 times per second, 1 cycleof the wave form of function generator 64 will be triggered. Since only20 of the 40 cycles of the wave form of the function generator aretriggered in any one second and since the die is only open for one halfof each triggered cycle, the ratio of the open time to the closed timeof the die is one to three. The use of the trigger signal adds greaterflexibility to the various combinations that may be employed for therelationship of the open to closed time of the die.

In the three alternate types of function generators described above, itis desirable to retain D.C. signal generator 66 whereby an adjustmentmay be made to vary the position of the wave form generated by signalgenerator 64 with respect to the ordinate to coincide with the actualclosed position of the dies with the abscissa of the selected generatedwave form.

In any of the alternate embodiments above described, the wave form maybe chopped for example by removing the wave form portion whichrepresents the die closed time period. In such case, the functiongenerator will not generate a signal during the die closed time period.This may be desirable to reduce wear of the dies due to excessivepressure between them during the die closed period.

During operation of the extrusion apparatus 10, the output of functiongenerator 64 is set to provide an amplitude and frequency in the waveform which correlates to a desired reciprocal motion of die member 18and then D.C. generator 66 is adjusted to correlate the abscissa to thebarely closed position of the die. The resulting operating signal istransmitted through switch 68 to servo amplifier 70 where it isamplified and then transmitted to a torque motor 72 which oscillates inresponse and synchronizes the servo valve 44 to the operating signal.

As the wave form rises to its maximum amplitude in the first portion ofthe cycle (66A) thus dictating elevation of the die member 18 to itsmaximum height, the

servo valve 44 is controlled to switch itself internally to allow oil toflow from the valve into conduit 52A and from conduit 50A back into thevalve. As a result the high pressure (2,000 psi.) oil delivered byconduit 42 to the servo valve 44 is caused to flow into the space 52Bbelow the bottom face of piston 30 and out of the space 503 above thetop of piston 30 whereby the piston is elevated and the attached diemember 18 is also raised. As the wave in the first portion of the cycledecreases in amplitude, the lower voltage changes the direction of thetorque motor 72 whereby the servo valve operation is reversed.Specifically, the servo valve 44 switches internally to allow oil toflow from the valve into conduit 50A and from conduit 52A back into thevalve, therefore, the high pressure oil delivered by conduit 42 and tothe servo valve is caused to flow into the space 508 above the top faceof the piston and out of the space 523 below the piston 30, whereby thedie member 18 is depressed into its closed position in contact with diemember 12. The lower pressure oil delivered from pressure regulator 46into conduit 48 to the servo valve 44 and returned by conduit 62 to thereservoir 32 gives a hydraulic assist in conventional manner to themovement of the servo valve parts and does not flow into the hydrauliccylinder 28. This hydraulic assist helps to maintain the servo valveoperation uniform, particularly at high frequency operation.

From the foregoing, it will be appreciated that the reciprocation of thedie member 18 up and down vertically to accomplish a reciprocabledisplacement motion between this member and ring member 12 is carriedout directly by hydraulic fluid pressure acting on the double-facedpiston 30. The flow of oil into and out of the hydraulic cylinder 28 iscontrolled by electrical signals emanating from the function generator64 and the DC signal generator 66 and transmitted through switch 68,servo amplifier 70 and torque motor 72 to the servo valve 44.

In order to increase the precision of the reciprocable displacementmotion, the actual motion of the piston of die member 18 is monitoredand a signal corresponding to the actual motion is generated and fedback to the servo amplifier 70. An overhead shaft 74 which is attachedto the top face of piston 30 passes up through the top of hydrauliccylinder 28. The overhead shaft 74 is in turn connected to a crossbar 76which is connected to the core arm 78 of a direct current displacementtransducer 80 (Type series 24 DCDT available from Hewlett-Parkard). Thetransducer 80 has a coil assembly (not shown) which is energized with 24volt DC current and arranged to cooperate with a coaxial core (notshown) the position of which is determined by the connected core arm 78as it rises or falls relative to the coil assembly. Thus, as the diemember 18 reciprocates up and down vertically, the rigid mechanicalconnections 74, 76 and 78 cause the core to reciprocate with the samemotion as that of the member 18 in terms of amplitude and frequency. Theresulting displacements of the core along the axis and within the boreof the core assembly produce output voltage changes proportional to thedisplacements, whereby transducer 80 generates a signal corresponding tothe actual motion of die member 18 and this signal is fed throughconnector 86 into the servo amplifier 70.

FIG. 3 illustrates a wave form of the signal that may be generated bythe transducer 80 in response to movement of die member 18 for the sinewave of FIG. 2. The

wave form of FIG. 3 will necessarily be similar to the wave formgenerated by function generator 64. As will be noted in the die openportion of the cycle, the signal corresponds to the die open portion ofthe wave form signal transmitted to the servo amplifier 70 from thegenerators 64 and 66. However, in the die closed portion of the cycle,the core arm 78 and consequently the connected core remains stationarywithin the coil assembly of transducer 80 and therefore a steady outputvoltage emanates from the transducer into the line 86 when member 18 isclosed against die member 18. This differs from the operating wave formsignal shown in FIG. 2 and the difference is used to advantage in a wayto be described below.

When the signal from transducer 80 is fed into servo amplifier 70, it iscompared electrically with the operating signal transmitted from thegenerators 64 and 66. If the signals are different, the output of theservo amplifier 70 is automatically adjusted to provide compensation, soas to eliminate the difference and tend to restore the combined outputsignal of the amplifier to that corresponding to the wave formtransmitted from generators 64 and 66. More specifically, if at anypoint along the time axis the voltage of the FIG. 3 signal is lower thanthat of the FIG. 2 signal, then the gain of the servo amplifier 70 isautomatically increased to compensate for the detected difference andthereby restore the motion of die member 18, and concommitantly thevalue of the FIG. 3 signal, to that of the FIG. 2 signal, Conversely, ifthe compared voltage values at any point along the time axis detect thatthe FIG. 3 signal is higher than the FIG. 2 signal, then the gain of theservo amplifier 70 is automatically decreased to compensate for thedetected difference in an opposite direction and again thereby restorethe motion of die member 18 and concommitantly the value of the FIG. 3signal to that of the FIG. 2 signal. In this way, deviations of themotion of the die member 18 from the motion dictated by the FIG. 2signal are constantly monitored and compensated, so that a variablethird output signal of the servo amplifier 70 is generated continuouslyand fed to the torque motor 72 for controlling the motion of the pistonand die member 18 to that motion which is dictated by the FIG. 2 signal.

As previously mentioned, in the die closed portion of the cycle, thevalue of the FIG. 3 signal will always vary considerably from that ofthe FIG. 2 signal, since the die member 18 physically cannot move downto a position lower than its closed contacting position on thestationary ring member 12. Therefore, in the die closed portion of thecycle, the servo amplifier 70 will always increase its gain and attemptto compensate for this large variation. As a result, additional pressurewill be generated on the top of piston 30. In this way, the closingpressure of the die members 18 and 12 is increased to insure that thedies will be tightly sealed to allow only the spaced verticallongitudinal strands 16 to be extruded, without polymer leakage betweenthe other contacting surfaces of the dies. If desired, this may beachieved with the alternate function generator described previously bysimply adjusting DC generator 66 to lower the generated wave form inrelation to the ordinate whereby there will be a difference between thegenerated signal and the signal generated from transducer 80 to increasethe pressure upon the top of the piston 30 during the die closed time.In such case, there will be a change in the open to closed timerelationship of the die members as described above.

The normal operation of the system described above is carried out bymaintaining switch 68 in its full line position shown in FIG. 1. In amodified embodiment of the invention, the switch 68 may also assume analternate position shown by the dotted lines. In this position, the waveform signal of the combined outputs of function generator 64 and DC.generator 66 is fed into a conventional timer 88 by means of line 89 andfrom the timer through line 90 to switch 68 and the servo amplifier 70.Timer 88 is also connected to a second function generator 64A and to asecond DC generator 67 each of which may feed an output signal intotimer 88.

The function generator 64A may be of any type similar to functiongenerator 64 and the alternate embodiment thereof. Further, functiongenerator 64A need not be the same as the particular embodiment beingused as function generator 64 and the function generator 64A may also beeliminated entirely with the retention of DC signal generator 67 byitself whereby only a DC function from DC generator 67 instead of a waveform is fed to timer 88. The timer 88 may be set to alternately switchto each of the desired input signals and transmit such signals for aspecified time period as output signals into line 90 and timer 80 may beset to transmit any combination of the input signals. The signals fedinto switch 68 by timer 80 will control the movement of the die membersas previously described hereinabove.

FIG. 4 illustrates the type of wave signals that may be employed tocontrol movement of the die members. Signal A is a sine wave generatedby function generator 64' and DC generator 66 while signal B is also asine wave of greater amplitude and period generated by functiongenerator 64A and DC generator 67. Timer 88 is set to switch back andforth between signal A and signal B.

The product resulting from such a mode of operation is illustrated inFIG. 5. As there shown, the product consists of a first portion of netstructure having spaced longitudinal vertical strands l6 and integraltransverse strands 22 having a radial thickness corresponding to theamplitude of the wave of signal A of FIG. 4. The extrusion directionlength of the strand and spacing between strands in the direction ofextrusion will correspond to the die open and closed portions of thewave of signal A of FIG. 4. The second portion of the integral netstructure has the spaced longitudinal vertical strands l6 and integraltransverse strands 22 with a radial thickness and length and spacing inextrusion direction corresponding respectively to the amplitude andperiod of the wave of Signal B of FIG. 4. The length of the first andsecond portion of net structure of FIG. 5 is determined by the settingused in timer 88 and although the product of FIG. 5 illustrates equaltime periods for signal A and signal B, the timer 88 may be set toswitch at unequal times. It will be seen that the length in extrusiondirection of the transverse strands of signal B is greater than thelength of the transverse strands of signal A. Also, there are only twotransverse If for instance, a curve follower function generator is usedas function generator 64 to generate signal A and the generated waveform is of a decreasing period as depicted in FIG. 6 and generator 64Ais chosen, for instance to generate a chopped sine wave as signal B, andthe timer 88 is set to switch from A to B at equal periods of time, thenthe product will be that depicted in FIG. 7. As will be seen, theproduct consists of a first portion of net structure corresponding tosignal A of FIG. 6 in which the integral transverse strands 22 areunequally spaced from each other and a second portion i corresponding tosignal B of FIG. 6 in which the integral transverse strands 22 are ofgreater length in the direction of extrusion and equally spaced fromeach other in the net. If a function generator capable of generating avariable signal is selected from the group of function generatorsdescribed above for generator 64 then the net structures of FIGS. and 7may be produced without the need of second function generator 64A or DCgenerator 67.

The signal wave of another embodiment is shown in FIG. 8 in whichfunction generator 64A is not used and DC. generator 67 is used togenerate signal B, which is used in conjunction with signal A ofgenerator 64 and 66 by means of timer 88. In this case, the D.C. signalB is a steady voltage which holds the die member 18 in a fixed positionat a height above the ring die member 12 corresponding to the amplitudeof the steady voltage shown as a straight line of Signal B in FIG. 8.Dur ing this time, a continuous sheet of molten polymer material isextruded from the continuous open annulus defined by the elevated diemember 18 above the ring die member 12. The length of this sheet inextrusion direction is controlled by the period of time over which thetimer 88 is set to transmit the signal B from generator 67. When the endof that time is reached, the timer 88 switches back again totransmission of the combined signal from wave form generator 64 and DCgenerator 66, into the line 90, whereby reciprocation of the piston diemember 18 is resumed. In this embodiment, timer 88 is preferably set totransmit signal A for a period of about 0.03 to seconds and to transmitsignal B for a period of about 0.015 to 2 seconds.

The produce resulting from such a mode of operation is illustrated inFIG. 9. As will be seen, the product consists of alternating portions ofnet structure having the spaced longitudinal vertical strands l6 andintegral transverse strands 22 and a solid sheet of plastic material 98,which portions correspond respectively to the alternate signal outputsof timer 88 fed into the line 90. In the product illustrated in FIG. 9,the alternating portions of net and of solid sheet have equal lengths inthe direction of extrusion, owing to the setting of the timer 88 toequal alternate time periods and the resulting equal time periods ofduration of the reciprocal and straight line wave forms in FIG. 8.However, by variation of the settings of the timer 88 many other shapesand sizes of plastic net like products can be made.

For example, the timer 88 may be set to transmit the reciprocal signalfor a shorter time period, than the steady DC. signal, whereby theduration of the alternate reciprocal wave forms in the signal will beless than the alternate straight line wave forms. The structure of theproduct from this mode of operation is illustrated in FIG. 10 andconsists of the alternate net portions 100 and alternate solid sheetportions 102, with the lengths of the net portions being shorter thanthe lengths of the solid sheet portions in the direction of extrusion.Conversely, if the timer 88 is set to transmit the reciprocal signal fora longer time period than the steady DC. signal, the reciprocal waveform portions of the signal will predominate in time duration over thestraight line portions. The structure of the resulting product isillustrated in FIG. 11 and again consists of alternating net portions104 and solid sheet portions 106 but with the lengths of the netportions being longer than the lengths of the solid sheet portions inthe direction of extrusion.

Other variations of structure may be made by correlating the frequencyof the wave form portion of the signal with the settings of timer 88.For example, if the wave form generator 64 is set to generate asinusoidal wave at the frequency of one cycle per second, the timeduration of the first half of each wave in each cycle will be 0.5second. If the timer 88 is set to transmit the reciprocal signal for onesecond and the steady DC signal for less than 0.5 second, then a plasticnet will be extruded having transverse strands of alternating larger andsmaller cross section corresponding respectively to the longer open timeof die member 18 dictated by the 0.5 second portion of the reciprocalwave form, which is allowed to be operative once each second by thetimer 88, and the shorter open time of die member 18 dictated by theshorter period of the straight line portion transmitted by the timer.Such a product is illustrated in FIG. 12 and consists of the largercross section transverse strands 108 alternating with smaller crosssection transverse strands 110.

Further structural forms may be achieved with other correlations ofreciprocal wave form frequency and settings of timer 88. For example, ifthe wave form generator 64 is set to generate a sinusoidal wave at afrequency of 3 cycles per second, then the time duration of each of thefirst halves of the three waves comprising the three cycles will be0.167 second. If timer 88 is again set to transmit the reciprocal signalfor one second and the steady DC signal for more than 0.25 second, thena plastic net will be extruded having alternately two relatively smallcross section transverse strands and one relatively large cross sectiontransverse strand. Such a structure is illustrated in FIG. 14, referencenumeral 116 denoting the relatively small cross section transversestrands which are extruded during elevation of the die member 18 for0.25 second each time in one second of operating during transmission ofthe reciprocal signal by the output of timer 88, and reference numeral118 denoting the alternate signal relatively large cross sectiontransverse strand which is extruded during elevation of the die member18 for longer than 0.25 second during transmission of the steady DCvoltage.

The invention has now been described in tenns of its operatingprinciples as illustrated by specific embodiments thereof. The inventionprovides a means and method for operating an extrusion apparatus of thetype described with exceptional precision and control whereby productsof varying size, shape and dimension can be made continuously and withgreat uniformity. Additionally, entirely new products are achieved whichwere not previously known to the art and hence the structure of suchproducts is also comprehended by the invention.

While it is preferred to employ a hydraulic system for reciprocating oneof the die members, it will be understood that other means may beemployed for generating the reciprocating action of the member and insuch case the actual position of the two die members may be sensed bymeans of the above described displacement transducer or equivalentsthereof and fed back to the programmed input in order to compare theactual position of the die members to the programmed position and tomake the corrections necessary to eorrolate the actual position of thedie members to the programmed position.

It will be understood that it is intended to cover all changes andmodifications of the preferred embodiments of the invention hereinchosen for the purpose of illustration which do not depart from thespirit and scope of the invention.

What is claimed is:

1. In an extrusion apparatus for manufacture of plastic net likestructures, having two die members which are reciproeably displaceablerelative to each other be tween two different positions, in one of whicha plurality of spaced streams of flowing molten thermoplastic materialis extruded and in the other of which an additional stream of saidmaterial integral with and transverse to said spaced streams isextruded, the improvements comprising:

a. means for holding one of said die members in a stationary position;

b. means for moving the other said die member to displace it relative tosaid stationary die member between said two positions, said moving meansineluding:

l. a hydraulic cylinder,

2. a double-faced piston within said cylinder and connected to saidmovable die member,

3. means for applying fluid pressure upon one face of said piston tomove said movable die member in one direction and for applying fluidpressure to the opposite face of said piston to move said movable diemember in reverse direction said means for applying fluid pressurecomprising i. means for generating an electrical signal,

ii. comparator means for comparing the generated electrical signal withan electrical signal indicating the actual motion of the die whereby anadjusted signal is generated by said comparator means,

iii. means comprising servo valve means for converting said adjustedsignal to a hydraulic signal, and

iv. hydraulic pump means independent of said generated electricalsignal, said electrical signal indicating the actual motion of the die,and said adjusted signal for communicating hydraulic fluid underconstant pressure to said servo valve; and

c. generator means for generating said electrical signal indicating theactual motion of the die, said generator means comprising a transducerconnected to said double-faced piston; and

d. means for transmitting said electrical signal indicating the actualmotion of the die from the transducer to the comparator means.

2. In an extrusion apparatus for manufacture of plastic net likestructures, having two die members which are reciprocably displaceablerelative to each other between two difierent positions, the improvementcom prising:

a. means for holding one of said die members in a stationary position;

b. means for moving the other said die member to displace it relative tosaid stationary die member between said two positions, said moving meansincluding:

l. a hydraulic cylinder,

2. a double-faced piston within said cylinder and connected to saidmovable die member,

3. means for applying fluid pressure upon one face of said piston tomove said movable die member in one direction and for applying fluidpressure to the opposite face of said piston to move said movable diemember in reverse direction said means for applying fluid pressurecomprising i. means for generating an electrical signal,

ii. comparator means for comparing the generated electrical signal withan electrical signal indicating the actual motion of the die whereby anadjusted signal is generated by said comparator means,

iii. means comprising servo valve means for converting said adjustedsignal to a hydraulic signal, and

iv. hydraulic pump means independent of said generated electricalsignal, said electrial signal indicating the actual motion of the die,and said adjusted signal for communicating hydraulic fluid underconstant pressure to said servo valve; and

c. generator means for generating said electrical signal indicating theactual motion of the die, said generator means comprising a transducerconnected to said double-faced piston; and

d. means for transmitting said electrical signal indicating the actualmotion of the die from the transducer to the comparator means.

3. The apparatus of claim 2 in which said hydraulic pump means deliverstwo independent pressure hydraulic fluids to said servo valve means.

4. The apparatus of claim 1 in which said hydraulic pump means deliverstwo independent pressure hydraulic fluids to said servo valve means.

1. In an extrusion apparatus for manufacture of plastic net likestructures, having two die members which are reciprocably displaceablerelative to each other between two different positions, in one of whicha plurality of spaced streams of flowing molten thermoplastic materialis extruded and in the other of which an additional stream of saidmaterial integral with and transverse to said spaced streams isextruded, the improvements comprising: a. means for holding one of saiddie members in a stationary position; b. means for moving the other saiddie member to displace it relative to said stationary die member betweensaid two positions, said moving means including:
 1. a hydrauliccylinder,
 2. a double-faced piston within said cylinder and connected tosaid movable die member,
 3. means for applying fluid pressure upon oneface of said piston to move said movable die member in one direction andfor applying fluid pressure to the opposite face of said piston to movesaid movable die member in reverse direction said means for applyingfluid pressure comprising i. means for generating an electrical signal,ii. comparator means for comparing the generated electrical signal withan electrical signal indicating the actual motion of the die whereby anadjusted signal is generated by said comparator means, iii. meanscomprising servo valve means for converting said adjusted signal to ahydraulic signal, and iv. hydraulic pump means independent of saidgenerated electrical signal, said electrical signal indicating theactual motion of the die, and said adjusted signal for communicatinghydraulic fluid under constant pressure to said servo valve; and c.generator means for generating said electrical signal indicating theactual motion of the die, said generator means comprising a transducerconnected to said double-faced piston; and d. means for transmittingsaid electrical signal indicating the actual motion of the die from thetransducer to the comparator means.
 2. a double-faced piston within saidcylinder and connected to said movable die member,
 2. a double-facedpiston within said cylinder and connected to said movable die member, 2.In an extrusion apparatus for manufacture of plastic net likestructures, having two die members which are reciprocably displaceablerelative to each other between two different positions, the improvementcomprising: a. means for holding one of said die members in a stationaryposition; b. means for moving the other said die member to displace itrelative to said stationary die member between said two positions, saidmoving means including:
 3. means for applying fluid pressure upon oneface of said piston to move said movable die member in one direction andfor applying fluid pressure to the opposite face of said piston to movesaid movable die member in reverse direction said means for applyingfluid pressure comprising i. means for generating an electrical signal,ii. comparator means for comparing the generated electrical signal withan electrical signal indicating the actual motion of the die whereby anadjusted signal is generated by said comparator means, iii. meanscomprising servo valve means for converting said adjusted signal to ahydraulic signal, and iv. hydraulic pump means independent of saidgenerated electrical signal, said electrical signal indicating theactual motion of the die, and said adjusted signal for communicatinghydraulic fluid under constant pressure to said servo valve; and c.generator means for generating said electrical signal indicating theactual motion of the die, said generator means comprising a transducerconnected to said double-faced piston; and d. means for transmittingsaid electrical signal indicating the actual motion of the die from thetransducer to the comparator means.
 3. The apparatus of claim 2 in whichsaid hydraulic pump means delivers two independent pressure hydraulicfluids to said servo valve means.
 3. means for applying fluid pressureupon one face of said piston to move said movable die member in onedirection and for applying fluid pressure to the opposite face of saidpiston to move said movable die member in reverse direction said meansfor applying fluid pressure comprising i. means for generating anelectrical signal, ii. comparator means for comparing the generatedelectrical signal with an electrical signal indicating the actual motionof the die whereby an adjusted signal is generated by said comparatormeans, iii. means comprising servo valve means for converting saidadjusted signal to a hydraulic signal, and iv. hydraulic pump meansindependent of said generated electrical signal, said electrial signalindicating the actual motion of the die, and said adjusted signal forcommunicating hydraulic fluid under constant pressure to said servovalve; and c. generator means for generating said electrical signalindicating the actual motion of the die, said generator means comprisinga transducer connected to said double-faced piston; and d. means fortransmitting said electrical signal indicating the actual motion of thedie from the transducer to the comparator means.
 4. The apparatus ofclaim 1 in which said hydraulic pump means delivers two independentpressure hydraulic fluids to said servo valve means.